To enlarge the area of an intake passage for each combustion chamber of an engine without enlargement of an intake valve, internal combustion engines provided with two intake ports per combustion chamber have been finding utility in recent years. In such an internal combustion engine, an air-fuel mixture is caused to flow into each combustion chamber through two intake ports.
Further, as means for improving combustion in an internal combustion engine, it is effective, for example, to produce a vertical swirl, i.e., a so-called tumble flow (tumble swirl) F (Fa,Fm) in a cylinder in the intake stroke as shown in FIGS. 29 and 30.
For example, FIGS. 29 and 30 show the structure of one of cylinders of a 2-intake-port internal combustion engine designed to produce such a tumble flow Fa or Fm. In the drawings, there are depicted a cylinder block 22, a cylinder bore 24, a piston 26, a cylinder head 28, and a combustion chamber 30. Also illustrated are a pentroof 34 formed in an upper wall of the combustion chamber 30, and two intake passages 40',42' provided with each cylinder. An intake port 44' of each of the intake passages 40',42' is provided with an intake valve 58.
The pentroof 34 is provided with such an inclined wall that can guide an intake air flow, which has been introduced from each intake passage 40' or 42', downwardly along an inner wall of the cylinder bore 24, said inner wall lying on an extension of an axis of the intake passage 40' or 42' While also assisted owing to the guidance by the pentroof 34, the intake air flow from the intake passage 40' or 42' therefore advances in the direction of such a tumble flow as indicated by arrow Fa or Fm.
In the illustrated example, only one of the intake passages, i.e., only the intake passage 42' is provided with an injector 12. A spark plug 10 is disposed in the vicinity of the intake port 44' of the intake passage 42' equipped with the injector 12.
To promote the tumble flow, the shape of the intake port 44' is important. In general, it is devised to straighten the flow by forming the intake port 44' into a linear, i.e., straight port as shown in FIGS. 29 and 30 or by constricting the intake port 44' as depicted in FIG. 33. In FIGS. 29 and 30, symbols 40F,42F indicate conventional intake ports which are not straight ports.
Although the cross-section of such an intake port 44' is generally formed in a circular shape as shown in FIG. 31, it may also be formed in a substantially square shape in addition to such an elliptical or oval shape as depicted in FIG. 32.
A tumble flow (tumble swirl) produced as described above is effective in increasing the flame propagation velocity and the combustion stability. Illustrative experimental data of heat release Q, cylinder pressure P and heat release rate dQ are presented in FIG. 34. Compared with the standard (the conventional construction in which no tumble flow is produced intentionally), it is understood that a tumble swirl (i.e., formation of a tumble flow) produces smaller cyclic variations in heat release Q, cylinder pressure P and heat release rate dQ and exhibits better combustion stability.
In the drawings, numeral 47 indicates an exhaust port provided in communication with an exhaust passage 60 while numeral 59 designates an exhaust valve.
Where the intake port 44' has a circular or elliptical, cross-sectional shape, formation of the intake port 44' as a straight port to make the tumble flow stronger, however, results in the structure that the intake port 44' extends at an acute angle relative to a valve seat 62 and the cross-sectional area of the flow passage obviously becomes smaller. Further, constriction of the intake port 44' as depicted in FIG. 33 obviously results in a reduction in the cross-sectional area of the flow passage, leading to a reduction in the maximum flow rate. In other words, the strength of a tumble flow (the strength of tumbling) and the maximum flow rate (flow coefficient) are in such a contradictory relationship that the latter decreases as the former increases. Such a decrease in the maximum flow rate leads to a reduction in the full-open performance of the engine and, hence, is not preferred.
To strengthen the tumble flow, incidentally, the flow rate (flow velocity) of a flow on a side of the tumble flow, aid flow being an upper flow in the intake port relative to a central axis of the intake valve 58 as a boundary (see arrow a in FIGS. 29 and 33), is required to be greater than the flow rate (flow velocity) of a flow on an opposite side (see arrow b in FIGS. 29 and 33).
If such an imbalance in flow rate (flow velocity) can be positively developed, the strength of a tumble flow (the strength of tumbling) can be increased without lowering the maximum flow rate (flow coefficient).
With a view toward achieving reductions in both vibrations and gas mileage by operating an internal combustion engine on an air-fuel mixture leaner than the stoichiometric air/fuel ratio, internal combustion engines equipped with two intake ports and fed with an air-fuel mixture through both the intake ports have also been proposed in recent years. As a spark plug is positioned between the two intake ports in the above case, operation on a small amount of fuel is accompanied by the potential problem that the ignition may be impaired. This has posed the inconvenience that the engine is difficult to operate on a smaller amount of fuel.